Rising, falling, and change triggers in Arduino - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does a rising trigger detect in Arduino interrupts?

A rising trigger detects when a signal changes from LOW (0 volts) to HIGH (usually 5 volts). It's like noticing when a light turns on.

beginner

Explain a falling trigger in Arduino interrupts.

A falling trigger detects when a signal changes from HIGH to LOW. Imagine noticing when a light turns off.

beginner

What is a change trigger in Arduino interrupts?

A change trigger detects any change in the signal, either from LOW to HIGH or HIGH to LOW. It's like noticing any switch flip.

intermediate

How do you attach an interrupt with a rising trigger in Arduino code?

Use attachInterrupt(digitalPinToInterrupt(pin), ISR, RISING); where ISR is your interrupt function.

intermediate

Why use interrupts with rising, falling, or change triggers instead of checking pins constantly?

Interrupts let the Arduino react immediately to signal changes without wasting time checking pins all the time. This saves processing power and makes your program efficient.

Which trigger detects when a signal goes from LOW to HIGH?

A Rising trigger

B Falling trigger

C Change trigger

D No trigger

A rising trigger detects the signal changing from LOW to HIGH.

What does a falling trigger detect?

A Signal stays HIGH

B Signal changes from HIGH to LOW

C Signal changes from LOW to HIGH

D No change in signal

A falling trigger detects when the signal changes from HIGH to LOW.

Which trigger reacts to both rising and falling edges?

A Change trigger

B Falling trigger

C Rising trigger

D None

A change trigger reacts to any change, either rising or falling.

What Arduino function is used to attach an interrupt?

A pinMode()

B digitalRead()

C attachInterrupt()

D interruptAttach()

attachInterrupt() is used to set up interrupts on pins.

Why are interrupts useful compared to checking pins in a loop?

A They disable other functions

B They waste power

C They slow down the program

D They allow immediate reaction to signal changes

Interrupts let the Arduino react immediately without constantly checking pins.

Describe the difference between rising, falling, and change triggers in Arduino interrupts.

Think about how a light switch turning on or off relates to these triggers.

Explain why using interrupts with these triggers can make your Arduino program more efficient.

Compare checking a door constantly versus having a doorbell that rings when someone arrives.

Practice

1. Which interrupt trigger type activates when a signal changes from LOW to HIGH on an Arduino pin?

easy

A. RISING

B. FALLING

C. CHANGE

D. LOW

Solution

1. Step 1: Understand the meaning of RISING trigger

   The RISING trigger activates when the signal goes from LOW (0) to HIGH (1).

2. Step 2: Compare with other triggers

   FALLING triggers on HIGH to LOW, CHANGE triggers on any change, LOW triggers when signal is LOW.

3. Final Answer:

   RISING -> Option A

4. Quick Check:

   LOW to HIGH = RISING [OK]

Hint: RISING means signal goes up from LOW to HIGH [OK]

Common Mistakes:

• Confusing RISING with FALLING
• Thinking CHANGE triggers only on RISING
• Mixing LOW trigger with RISING

2. Which of the following is the correct syntax to attach an interrupt on pin 2 for a FALLING trigger in Arduino?

easy

A. attachInterrupt(2, ISR, RISING);

B. attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING);

C. attachInterrupt(2, ISR, CHANGE);

D. attachInterrupt(digitalPinToInterrupt(2), ISR, LOW);

Solution

1. Step 1: Identify correct function usage

   attachInterrupt requires pin number converted by digitalPinToInterrupt for portability.

2. Step 2: Match trigger type FALLING

   attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING); uses digitalPinToInterrupt(2) and FALLING correctly.

3. Final Answer:

   attachInterrupt(digitalPinToInterrupt(2), ISR, FALLING); -> Option B

4. Quick Check:

   Use digitalPinToInterrupt with FALLING [OK]

Hint: Use digitalPinToInterrupt(pin) with trigger type [OK]

Common Mistakes:

• Using pin number directly without digitalPinToInterrupt
• Mixing trigger types with wrong syntax
• Using LOW trigger incorrectly

3. What will be the output on the serial monitor if the following Arduino code is run and the input pin changes from LOW to HIGH, then HIGH to LOW?

volatile int count = 0;
void ISR() { count++; }
void setup() {
  Serial.begin(9600);
  pinMode(2, INPUT);
  attachInterrupt(digitalPinToInterrupt(2), ISR, CHANGE);
}
void loop() {
  Serial.println(count);
  delay(1000);
}

medium

A. 0 0 0 ... (no change)

B. 1 1 1 ... (count increments once)

C. 2 2 2 ... (count increments twice)

D. Error: ISR cannot increment variable

Solution

1. Step 1: Understand CHANGE trigger behavior

   CHANGE triggers on any signal change: LOW to HIGH and HIGH to LOW, so ISR runs twice.

2. Step 2: Count increments in ISR

   Each ISR call increments count by 1, so after two changes count = 2.

3. Final Answer:

   2 2 2 ... (count increments twice) -> Option C

4. Quick Check:

   CHANGE triggers twice, count = 2 [OK]

Hint: CHANGE triggers on both edges, count increments twice [OK]

Common Mistakes:

• Assuming CHANGE triggers only once
• Forgetting volatile keyword importance
• Thinking ISR cannot modify variables

4. Identify the error in this Arduino code snippet that tries to attach an interrupt on pin 3 for a RISING trigger:

void ISR() {
  // interrupt code
}
void setup() {
  attachInterrupt(3, ISR, RISING);
}

medium

A. ISR function must have parameters

B. attachInterrupt must be called in loop(), not setup()

C. RISING is not a valid trigger type

D. Pin number 3 should be converted using digitalPinToInterrupt()

Solution

1. Step 1: Check attachInterrupt pin argument

   Pin number must be converted with digitalPinToInterrupt(pin) for correct interrupt mapping.

2. Step 2: Validate other parts

   ISR has correct signature (no parameters), RISING is valid, attachInterrupt is correctly called in setup.

3. Final Answer:

   Pin number 3 should be converted using digitalPinToInterrupt() -> Option D

4. Quick Check:

   Use digitalPinToInterrupt for pin numbers [OK]

Hint: Always convert pin with digitalPinToInterrupt() [OK]

Common Mistakes:

• Passing raw pin number directly
• Thinking ISR needs parameters
• Calling attachInterrupt in loop()

5. You want to count how many times a button connected to pin 4 changes state (pressed or released). Which interrupt trigger should you use and why?

hard

A. CHANGE, because it counts both presses and releases

B. FALLING, because it counts only releases

C. RISING, because it counts only presses

D. LOW, because it triggers when button is held down

Solution

1. Step 1: Understand button state changes

   A button press changes signal from HIGH to LOW or LOW to HIGH depending on wiring.

2. Step 2: Choose trigger to count all changes

   CHANGE triggers on any signal change, so it counts both presses and releases.

3. Final Answer:

   CHANGE, because it counts both presses and releases -> Option A

4. Quick Check:

   Count all changes = use CHANGE trigger [OK]

Hint: Use CHANGE to count all button state changes [OK]

Common Mistakes:

• Using RISING or FALLING and missing half the events
• Confusing LOW trigger with change detection
• Not considering both press and release